Mesogen-Driven Formation of Triblock Copolymer Cylindrical Micelles

Evidences were gathered to support mesogen-driven formation of cylindrical micelles from BCF and ACF triblock copolymers. Here A, B, C, and F denote poly(acrylic acid), poly(tert-butyl acrylate), poly(2-cinnamoyloxylethyl methacrylate), and the liquid crystalline poly(perfluorooctylethyl methacrylate) block, respectively. At room temperature (21 degrees C) in all tested solvents that were selective for the A or B blocks, three of the four copolymers with various compositions formed exclusively cylindrical micelles possessing an F core, a C shell, and an A or B corona. Our further analyses indicated that the F core chains were almost fully stretched, and the C shell chains were compressed relative to their unperturbed dimensions. These abnormal chain packing motifs suggest that the need to form a liquid crystalline F phase in the cores dictated micelle formation and prevailed over the needs of the shell chains to achieve their normal stretched conformations. A subsequent wide-angle X-ray scattering study of the dried cylindrical micelles confirmed smectic A phase formation for the F blocks at room temperature. The smectic A to isotropic phase transition upon raising temperature was detected by a differential scanning calorimeter for the dry cylindrical micelles and by F-19 NMR for the solvated micelles. This smectic A to isotropic phase transition was accompanied by a morphological transformation from cylindrical micelles at room temperature to other morphologies at 70 degrees C. More interestingly, this cylinder to vesicle conversion could be cycled repeatedly by temperature cycling for one ACF sample.